DRILLING ENGINEERING: HOW DO DRILLING FLUIDS WORK?
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INTRODUCTION
Drilling fluids, also referred to as drilling mud, are added to the
wellbore to facilitate the drilling process by suspending cuttings, controlling
pressure, stabilizing exposed rock, providing buoyancy, and cooling and
lubricating.
Cuttings in Circulating Driling Fluids Source: Oil
& Gas UK
As early as the third century BC, the Chinese were
using drilling fluids, in the form of water, to help permeate the ground when
drilling for hydrocarbons. The term "mud" was coined when at
Spindletop in the US, drillers ran a herd of cattle through a watered-down
field and used the resulting mud to lubricate the drill.
While the technology and chemistry of
drilling fluids have become much more complex, the concept has remained the
same. Drilling fluids are essential to drilling success, both maximizing
recovery and minimizing the amount of time it takes to achieve first oil.
Purposes of Drilling Fluid
During drilling, cuttings are obviously created, but
they do not usually pose a problem until drilling stops because a drillbit
requires replacement or another problem. When this happens, and drilling fluids
are not used, the cuttings then fill the hole again. Drilling fluids are used
as a suspension tool to keep this from happening. The viscosity of the drilling
fluid increases when movement decreases, allowing the fluid to have a liquid
consistency when drilling is occurring and then turn into a more solid
substance when drilling has stopped. Cuttings are then suspended in the well
until the drill is again inserted. This gel-like substance then transforms
again into a liquid when drilling starts back up.
Drilling fluids also help to control pressure in a
well by offsetting the pressure of the hydrocarbons and the rock formations.
Weighing agents are added to the drilling fluids to increase its density and,
therefore, its pressure on the walls of the well.
Another important function of drilling fluids is rock
stabilization. Special additives are used to ensure that the drilling fluid is
not absorbed by the rock formation in the well and that the pores of the rock
formation are not clogged.
The longer the well, the more
drill pipe is needed to drill the well. This amount of drill pipe gets heavy,
and the drilling fluid adds buoyancy, reducing stress.
Additionally, drilling fluid helps to reduce friction with the rock formation,
reducing heat. This lubrication and cooling helps to prolong the life of the
drillbit.
Drilling Fluids Source: OSHA
Types of Drilling Fluids
Drilling fluids are water-, oil- or synthetic-based,
and each composition provides different solutions in the well. If rock
formation is composed of salt or clay, proper action must be taken for the
drilling fluids to be effective. In fact, a drilling fluid engineer oversees
the drilling, adding drilling fluid additives throughout the process to achieve
more buoyancy or minimize friction, whatever the need may be.
In addition to considering the chemical composition
and properties of the well, a drilling fluid engineer must also take
environmental impact into account when prescribing the type of drilling fluid
necessary in a well. Oil-based drilling fluids may work better with a saltier
rock. Water-based drilling fluids are generally considered to affect the environment
less during offshore drilling.
Disposal of drilling fluids
after they are used can also be a challenge. Recent technological advances have
established methods for recycling drilling fluids.